High convergence electron gun with magnetically shielded cathode



June 7, 1966 D. v. GEPPERT 3,255,370

HIGH CONVERGENCE ELECTRON GUN WITH 1 MAGNETICALLY SHIELDED CATHODE 2 Sheets-Sheet 1 Filed NOV. 17, 1961 INVENTOR. DONOVAN V. GEPPERT Y MM J TTORNEY June 7, 1966 Filed Nov. 17, 1961 HIGH CONVERGENCE ELECTRON GUN WITH 2 Sheets-Sheet 2 Q 5? d 100-- L[ E 90 r I 9 58 E 80- I 5 l 70- 2 I 60- 5 g 50- i 0 40 B 2 30- 0 "3-4 2 IO- l o E mm 0 I I I I I I f 4 -.5 o .5 L5 2 3 4 z DISTANCE-INCHES /)i A 4e\, A f D INVENTOR.

DONOVAN V. GEP PERT ATTORNEY United States Patent 3,255,370 HIGH CONVERGENCE ELECTRON GUN WITH MAGNETICALLY SHIELDED CATHODE Donovan V. Geppert, Sunnyvale, Calitl, assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Nov. 17, 1961, Ser. No. 152,985 3 Claims. (Cl. 313-84) This invention relates to electron guns and more particularly to the forming and shaping of emitted electrons into a high density beam of small diameter.

Two widely used techniques for producing and confining electron beams in traveling wave tubes are known as the confined flow and Brillouin flow systems. In the confined flow operation, electrons comprising the beam are immersed in a strong longitudinal magnetic field provided by a solenoid or permanent magnet. This type of flow is generally produced by a plane-type emitter (fiat emitting surface) and the current density of the resultant beam, which is a measure of the power of the tube, is equal to that of the emitter.

In the Brillouin flow system a cathode with a concave emitting surface is used to produce electrons which are accelerated and formed into a converging beam by a set of focusing electrodes. That is, the diameter of the beam is progressively reduced as it travels toward the start of the slow wave structure of the tube. In a plane beyond the last gun electrode, the beam achieves a minimum diameter; at which time it is abruptly introduced into an axially oriented magnetic field. For convenience, the term abruptly is used to indicate the entrance condition encountered by the beam in passing from a region free of external fields to one having a magnetic field in the axial direction only. (See L. Brillouin, A Theorem of Larmor and Its Importance for Electrons in Magnetic Fields, Physical Review, volume 67, No. 7 and 8, April 1 and15, 1945.)

Although higher density beams are obtainable from a concave emitter than from a plane emitter of the same diameter, the former involves a problem that the amount of convergence of the beam, and hence electron density, directly relates to the focusing characteristics of the electron gun. More particularly, the amount of convergence of the beam depends upon the magnitude and orientation of the electrostatic fields provided by the focusing electrodes. Additional convergence by inward focusing forces derived from the interaction of the electrons comprising the beam with a gradually increasing magnetic field has not been used because of the unpredictable relationship between the diameter of the electron beam and the magnetic field in the region where the latter is increasing.

In accordance with the invention, Brillouin focused elec tron beams of small diameter are achieved by providing a gradually increasing axial magnetic field in the region between the gun structure and the commencement of the slow wave structure. Inward focusing forces derived from the interaction of the electrons of the beam with the longitudinal component of the field continue the convergence initiated by the electrostatic forces of the focusing electrodes causing the radius of the electron beam to decrease exponentially in the direction of electron flow to a value which sustains Brillouin flow. The increasing field is an ancillary portion of the longitudinal magnetic fields produced by an electromagnetic focusing solenoid surrounding the beam and is provided by a cylindrically shaped pole piece extension or hub .attached to and integral with an end of the solenoid.

It is therefore an object of the invention to provide an improved electron gun for traveling wave tubes wherein electron beams of high current densities in Brillouin flow are realized.

Another object is to provide an electron gun having a spherically shaped cathode for production of small diameter beams wherein Brillouin flow is achieved.

A still further object of the invention is'to provide .an apparatus for producing in conjunction with a converging electron beam, a converging magnetic field which permits the beam to be focused to a small diameter which sustains Brillouin flow.

A still further object is to provide means for producing, in conjunction with a converging electron beam provided by an electron gun having a limited number of focusing electrodes, a controlled, gradually increasing magnetic field in the direction of beam travel which permits the beam to be focused to a small diameter which sustains Brillouin flow.

The above-mentioned objects and features of the invention will be more clearly understood from the following detailed description when read in conjunction with the drawings in which:

FIGURE 1 is a plan view, partially in cross-section and broken near each end, of a traveling wave tube and an electromagnetic solenoid illustrating the electron gun and the magnetic field shaping pole piece of the solenoid;

FIGURE 2 is a transverse section of the pole piece of the solenoid taken along line 22 of FIGURE 1;

FIGURE 3 is an enlarged perspective view of the pole piece taken along line 33 of FIGURE 1 illustrating the path of an electron within the pole piece under the influence of a beam-controlling magnetic field;

FIGURE 4 is a normalized plot of axial magnetic field strength of the solenoid as a function of location along its axis of symmetry comparing the magnetic field buildup curves of the piror art and that of the present invention;

FIGURE 5 is an enlarged view of the field shaping pole piece of the solenoid of FIGURE 1, partially broken away, and illustrates the distribution of a portion of the flux density of the magnetic field about the axis AA of the assembly, and

FIGURE 6 is a partially schematic view of the electron gun structure of the device shown in FIGURE 1 and illustrates the trajectories of a few of the electrons of the beam in the region between the cathode and the ordinate points S and T on the axis of beam travel.

Referring to FIGURE 1, a traveling wave tube generally indicated at 10 is provided with an external magnetic circuit, such as an electromagnetic solenoid 11, suitably positioned relative to the central region of the tube and extending parallel with the longitudinal axis AA of the former. The solenoid 11 consists of layers of wire, generally indicated at 12, electrically isolated from one another, wound about a centrally disposed tubular member 13, the walls of which define an envelope into which the tube 10 is located. An elongated cylindrical shell 14 protects the windings 12 from foreign matter and the like, and is mounted to annular pole pieces 16 and 17 located at respective ends of the member 13.

When .an appropriate energizing potential is applied to the winding, as by leads 18 protruding through pole piece 17, a magnetic field is produced. In the region adjacent the elongated barrel 19 of the envelope (FIGURE 2), this field is coextensive with the longitudinal axis of the tube to radially contain the beam with minimum magnetic field. In the region adjacent the spherically-concave surface 20 of the cathode 21, the field is substantially zero, but increases to the right as viewed, to provide a gradually increasing magnetic field in the direction of beam travel. The relationship of the field and beam in these respective regions will be made clearer by the mathematical analysis which follows.

In order to provide rectilinear motion of the electrons emanating from the surface 20, an electrostatic field forming means, such as focus cup 23, and anode 25 is arranged about the path of the electron beam adjacent the cathode. The focus cup 23 is formed of a cylinder having a centrally apertured end 26, located adjacent the surface of the cathode by its attachment with circumferentially spaced, axially extending ceramic supports 28.

Anode 2'5, coaxial with longitudinal axis AA, is comprised'of cylinder having a re-entrant lip 31. Axial and radial location of anode 25 relative to focus cup 23 and cathode 21 is provided by ceramic members 28 located at and permanently afiixed to its peripheral edge.

The surface 20 of cathode 21, as well as focus cup 23 and anode 25, are suitably energized by outside energy sources (not shown) connected by means of leads 33 located at the end of the gun envelope. In operation, the temperature of surface 20 is increased when the heater 34 located adjacent its inner surface is energized, which causes electrons to be emitted from surface 20. On application of a positive potential (relative to cathode 21) to anode 25 and a suitable focusing potential to focusing cup 23, a field pattern forms the emitted electrons into a conically-converging beam, successive segments of which are then accelerated toward the slow wave structure of the tube.

However, this converging field pattern is derived from electrostatic forces only, a portion of which is indicated at 35 in FIGURE 6 as imaginary surfaces of equal potential formed as a result of the static charges of focus cup 23 and anode 25. In the region adjacent the cathode 21, the curvature of these surfaces matches that of the surface 20 such that the respective centers of formation are coincident at the ordinate S located on the axis of beam travel (FIGURE 6), forming a smoothly converging beam in the region between the cathode and point S. Trajectories 38 of individual electrons comprising the beam depicting the smoothly converging nature of the beam in this region are shown in FIGURE 6.

Along successive planes normal to the beam axis in this region there is no tendency for the electrons to deviate from converging paths because the potential distribution outside the beam is similar to the potential.distribution existing inside the beam. Stated somewhat differently, the field forming means including the focus cup 23 and anode 25 effectively produce forces normal to the edge of the electron beam to balance the neutral space charge forces of the electrons.

Note that the field balances the space charge forces of the beam only over a finite axial distance, and that the distance depends on the number and spacing of the electrodes as well as the magnitude of the biasing voltage of the latter. This qualification has resulted in beams of small convergence being produced from electron guns having a limited number of electrodes and it is to such guns that this invention is directed.

As the beam passes the anode region, the converging effects of the electrostatic fields of the gun electrodes diminish and the space charge forces of the electrons comprising the beam become more predominant. However, these space charge forces are balanced and convergence of the beam continues owing to the fact that the beam is introduced into a gradually increasing magnetic field in the region between ordinate points S and T located on the axis of beam travel (FIGURE 5). This gradually increasing field is derived from the longitudinal magnetic field provided in the central region of solenoid 11, the shape of the field being dependent upon the shape and properties of magnetic flux shaping hub 40 extending from and integral with the pole piece 16 of the solenoid 11. The reasons for the convergence of the beam can be best explained with references to FIGURES 3 and 5.

When a voltage is applied to the windings of solenoid 11, a magnetic field is produced along the longitudinal axis of the tube. Owing to the aperture between the planes of surfaces 52 and 53 of the pole piece 16 of the solenoid 11, a fringing field is also created, the axial distribution of which is controlled by the physical shape and magnetic properties of hub 40. The interrelation of the hub 40 and the fringing field, in turn, provides an axially increasing series of flux lines which, when crossed, interact with the electrons comprising the beam to further converge the beam.

For example, as the electron beam enters the magnetic field at the plane including point S in FIGURES 5 and 6, the beam is moving in a converging path with respect to the longitudinal axis of the tube. As shown in FIG- URE 3, the travel of one such electron at the edge or boundary of the beam is represented by thick line 55 while thinlines 51 represent a portion of the magnetic lines of flux of the fringing field.

Owing to the outward fringing of the longitudinal field, a transverse component of magnetic field (radially inward) occurs at point S, FIGURE 5, which causes the electron beam to acquire an angular velocity. The electron, being a charged particle, is acted upon as it crosses the radially inward magnetic field and begins to spiral along path 56. As the electron crosses increasing values of flux, the angular velocity increases While the beam continues to converge exponentially to a certain value at the plane including ordinate point T, FIGURE 6, at which time Brillouin fiow occurs.

That is, when the beam reaches point T, its diameter is such that the electrons comprising a segment of the beam are in dynamic equilibrium with the longitudinal magnetic field. The inwardly directed forces resulting from the longitudinal component of the magnetic field and the rotational velocity of the electrons cancel the outward forces of centrifugal force (of rotation) and the space charge effects of the electrons.

In FIGURES, the increasing values of flux crossed by the beam are depicted as lines 51 which, as shown, follow diverging paths in the region between the planes including ordinate points S and T and terminate on the adjacent surfaces of hub 40, the zone of termination being concentrated in the planes defined between the forward and rearward surfaces 52 and 53, respectively, of the pole piece 16. Thus, the composite of these flux lines, axial magnetic field B (FIGURE 4) is a gradually increasing field in this region and, as developed mathematically hereinafter, relates to the magnetic field sustaining Brillouin flow, B and the normalized exponentially decreasing radius of the beam as follows:

& & aZ R V 1 Rf (1) where normalized quantities of this equation are related to actual quantities by the equations B is the axial magnetic flux density at any point along axis AA B is the fiux'density in Brillouin flow, given by the usual relationship B IO 1,2 f

m EtuSS OI T 1'1 Cm.

(W0) g I R is the normalized radius of the beam r is the actual beam radius at any point r is the radius in Brillouin fiow R is the value of R at 2:0 (at point S in FIGURE 6) on is a constant given by z is the actual axial distance from the zero magnetic field point (point S, FIGURE 6) Z is the normalized axial distance I is the beam current and V is the beam voltage (strictly speaking, the voltage on the beam axis) e is the permittivity of free space 1; is the electronic charge-to-mass ratio 3 p 18 the beam perveance: I /V E Referring now to FIGURES 1 and 6, hub 40 which provides the field in accordance with Equation 1 is cylindrically shaped, having a forward end integral with planar pole piece 16 and an axis colinear with the axis AA of beam travel. The inner surface 41 of the hub is reduced near rearward end 42 to form rearward and forward portions 43 and 44, respectively, with apertured extension lip 45 located therebetween for diminishing the magnetic field in the direction of the gun. The term forward is used, above, to describe the end of the assembly toward which the electrons travel after emission and refers to the right end of the hub 40 in FIGURE 1. Similarly, rearwarddesignates the left end of the hub, as viewed.

Rearward portion 43 fits coaxially about the gun envelope of the tube to provide radial location of the tube relative to the axis of the solenoid and has a rearward portion which extends beyond the end of the envelope to shield the electron gun from stray magnetic fields. The rearward portion 43 also has an inwardly protruding step at its forward end which forms rearward surface 48 of lip 45 (FIGURE 6) which contacts the forward wall 47 of the envelope thereby locating the electron gun relative to the pole piece 16 of solenoid 1'1.

Centrally disposed within lip 45, aperture 46 provides a wall 49 snugly fitting about the elongated barrel 19 of the tube. In addition to preventing radial movement of the tube relative to the solenoid 11, the' snug fit between the barrel and the wall of the aperture 46 also minimizes the amount of magnetic field which threads the cathode, as required to achieve Brillouin flow Coaxially located of the forward portion of the barrel of the tube to provide precise build-up of the axial magnetic field, forward portion 44 comprises an elongated cavity 50 of constant diameter having a forward end integral with pole piece 16. The forward portion 44 also has an inwardly protruding step at its rearward end which forms forward surface 54 of lip 45 (FIGURE 6) to provide a termination surface for the flux lines 51 (FIGURE constituting the magnetic field as explained above.

When a voltage is applied to the windings of the solenoid, the distribution of the magnetic field in the forward portion 44 of hub 40 is as illustrated in FIGURE 5. If care is taken to eliminate magnetic saturation of the hub during tube and solenoid operations, the axial distribution of the gradually increasing field depends only on the diameter of the aperture 46, and the diameter and axial length of portion 44. However, note that dimensions of these parts relate to one another such that changes in one can be compensated by changes in the remaining terms to achieve the desired field build-up.

By way of example of the correct relationship of the parts, a hub having the following dimensions has been constructed and tested with a traveling wave tube operating in X-band frequencies (8.0 to 12.5 kilomegacycles) in conjunction with a solenoid having a peak axial magnetic field of 3000 gauss; the assembly provided an axial magnetic field and fiux distribution which converged the electron beam-emanating from a cathode having a diameter of 0.470 inch and a density of 2 amperes per centimeter squared, to a final diameter of 0.0036 inch:

Dimensions Item in inches Portion 44:

Inside diameter a 1.325

Thickness 0.087

I Length 2.40 Lip 45:

Aperture 46 0.350 Thickness 0.060 Portion 43:

Diameter 1.405

Thickness 0.040

Length 3.00

A plot of the normalized axial magnetic field provided by the above described hub 40 as a function of axial distance Z from the plane including the ordinate points S and T, FIGURES 5 and 6, is presented in FIGURE 4. In the plot, the ratio B /B is the normalized axial magnetic field where B is the axial magnetic field, and

hate on or pass through the aperture.

B is the axial magnetic field in Brillouin flow. Ordinate points S and T (FIGURE 5) are also shown to relate the normalized field strengths as a function of axial distance from the point of zero magnetic field on the axis of beam travel, i.e., point S within the aperture 4-6 of hub 40.

To further understand this field plot, consider a pole piece with no aperture therein and having a planar pole face. A plot of the magnetic field strength along the axis of symmetry AA would be constant up to the pole face as shown by the dotted line 57 in the figure, whereat the magnitude of field would drop instantaneously from a given magnitude to zero, i.e., the field is a step function of distance Z along the axis AA of the assembly. It is this type of magnetic field build-up upon which earlier gun designs were postulated for use in Brillouin focused traveling wave tubes.

However, in practice, step function characteristics of the idealized magnetized magnetic field could not be realized'because of the leakage flux through the aperture of the pole piece. That is, the lines of force which would terminate on a plane pole face having an aperture termi- Some of the lines terminate inside the aperture, and some will crowd toward a plane situated adjacent the pole face. The net result is to change the field distribution so that the axial field plot is changed to the solid line curve 58 in FIGURE 4. This change in field distribution is compensated for by changes in the gun design which are usually done empirically.

However, in the instant invention, by forming of the hub 40 integral with the pole piece 16 and having the physical shape described above, the field is redistributed to the solid line curve 59. The curve illustrates that the axial magnetic field increases from Z=0 to 2:3 inches in a manner which converges the electron beam relative to the axis of beam travel and whereafter is constant for values of Z greater than 3 inches, to sustain the beam in Brillouin flow throughout the interaction region of the The radical force equation for an electron at radius r at the edge of the beam is 1+e B.11 -a (5) where i is the second derivative of radius with respect to time and 0' is the angular beam velocity.

The first term on the right is the space charge force component, where p (a positive number) is the charge density of the beam (assumed independent of radius out to the edge of the beam), and 1 (a positive number) is the electronic charge-to-mass ratio. The second term on the right is the centrifugal force component, where 0 is the angular velocity of the electron. The last term on the right is the inward magnetic force on the electron (B and (9 being positive quantities).

The charge density can be expressed in terms of the v beam current, beam voltage, and beam radius,

For shielded cathods, the angular volocity is related to the axial magnetic field B by Buschs theorem,

amt 2 7) we use 6 and 7 to rewrite 5 1/21 1 23 2 Mt/2v, 7 4

The time derivative in 8 can be converted into a space derivative yielding where r is the second derivative of r with respect to z.

Brillouin flow is obtained when the two terms on the right cancel, yielding Equation 1 for the Brillouin field B for radius r B W auss for r in cm r... 4 g m (10) the normalized trajectory equation becomes 2R"= -B R where R is the normalized radius defined by B is normalized axial magnetic field defined by where Z is the normalized axial distance, measured from the zero magnetic point S (see FIGURE 6), R being the value of R at Z=0, and on a constant given by a /2R(Rol) (t the value of R in Brillouin fiow being unity and therefore R being the radius convergence ratio, the normalized axial magnetic flux density B required is as presented above.

The foregoing analysis relates to an electron at the edge or boundary of the beam. Other electrons, nearer the axis of the beam, are subject to similar forces. They, however, cross less flux in the vicinity of the aperture and have lower transverse velocity in the uniform field and are subject to less total space charge force and less centrifugal force so they follow paths similar to those of the edge electrons but at smaller radii.

It should be understood that this invention in its broadest aspects is not limited to the specific example herein illustrated and described, and that the following claims are intended to include all changes and modifications within the spirit and scope of the invention.

What is claimed is:

1. An electron dishcarge device comprising a cylindrical vacuum envelope with one section having a diameter substantially greater than the diameter of an adjacent section and having a longitudinal axis,

a cathode for producing a converging electron beam along the axis, said cathode being located in said one section in a region substantially free of magnetic field,

a collector axially spaced from said cathode along the axis in the adjacent section for receiving the electron beam,

a coaxially disposed magnetic field generating means axially spaced from said cathode toward said collector and adapted to produce a magnetic field for focusing the beam, and

a coaxially disposed magnetic flux shapping hub member attached to the end of said magnetic field generating means proximate said cathode and extending therefrom over said cathode.

said magnetic flux shapingshub member having a cylindrical portion with a fiat lip projecting radially in- 'wardly therefrom immediately adjacent the junction of said sections of said cylindrical vacuum envelope,

said inwardly projecting lip having a coaxially formed cylindrical aperture proximate the electron beam and said second section and dividing the interior of said cylindrical portion into a first cavity facing said magnetic field generating means and a second cavity containing the cathode,

the transverse dimension of said first cavity being smaller than the transverse dimension of said second cavity.

2. The electron discharge device according to claim 1 wherein said magnetic field generating means comprises a solenoid.

3. The electron discharge device according to claim 1 wherein said magnetic flux shaping hub member has a flat lip projecting radially outward from said cylindrical portion immediately adjacent said magnetic field generating means.

References Cited by the Examiner UNITED STATES PATENTS 2,608,668 8/1952 Hines 313-84 X 2,687,490 8/1954 Rich et a1 313-84 X 2,707,758 5/1955 Wang 313-84X 2,797,353 6/ 1957 Molnar et a1.

1 0 2,941,111 6/1960 Veith et a1 3153.5 3,066,237 11/1962 Nevins 313-84 3,092,745 6/1963 Veith et a1. '31384 FOREIGN PATENTS 1,075,211 10/ 1954 France.

GEORGE N. WESTBY, Primary Examiner.

Assistant Examiners. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A CYLINDERICAL VACUUM ENVELOPE WITH ONE SECTION HAVING A DIAMETER SUBSTANTIALLY GREATER THAN THE DIAMETER OF AN ADJACENT SECTION AND HAVING A LONGITUDINAL AXIS, A CATHODE FOR PRODUCING A CONVERGING ELECTRON BEAM ALONG THE AXIS, SAID CATHODE BEING LOCATED IN SAID ONE SECTION IN A REGION SUBSTANTIALLY FREE OF MAGNETIC FIELD, A COLLECTOR AXIALLY SPACED FROM SAID CATHODE ALONG THE AXIS IN THE ADJACENT SECTION FOR RECEVING THE ELECTRON BEAM, A COAXIALLY DISPOSED MAGNETIC FIELD GENERATING MEANS AXIALLY SPACED FROM SAID CATHODE TOWARD SAID COLLECTOR AND ADAPTED TO PRODUCE A MAGNETIC FIELD FOR FOCUSING THE BEAM, AND A COAXIALLY DISPOSED MAGNETIC FLUX SHAPPING HUB MEMBER ATTACHED TO THE END OF SAID MAGNETIC FIELD GENERATING MEANS PROXIMATE SAID CATHODE AND EXTENDING THEREFROM OVER SAID CATHODE. SAID MAGNETIC FLUX SHAPING HUB MEMBER HAVING A CYLINDRICAL PORTION WITH A FLAT LIP PROJECTING RADIALLY INWARDLY THEREFROM IMMEDIATELY ADJACENT THE JUNCTION OF SAID SECTIONS OF SAID CYLINDRICAL VACUUM ENVELOPE, SAID INWARDLY PROJECTING LIP HAVING A COAXIALLY FORMED CYLINDRICAL APERTURE PROXIMATE THE ELECTRON BEAM AND SAID SECOND SECTION AND DIVIDING THE INTERIOR OF SAID CYLINDRICAL PORTION INTO A FIRST CAVITY OF SAID MAGNETIC FIELD GENERATING MEANS AND A SECOND CAVITY CONTAINING THE CATHODE, THE TRANSVERSE DIMENSION OF SAID FIRST CAVITY BEING SMALLER THAN THE TRANSVERSE DIMENSION OF SAID SECOND CAVITY. 